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Clues to Violence May Be Visible in the Brain

May 23, 2018

Aggression has destructive and painful impacts on society, as we have seen with mass shootings, domestic violence, and childhood bullying. It is strongly tied to mental health problems, and can manifest not only in antisocial personality and conduct disorders but also with posttraumatic stress disorder (PTSD), narcissistic personality disorder, alcohol dependence, and even anxiety and depression. But it appears that some common brain pathways underlie violent behaviors regardless of the co-occurring conditions, according to Edelyn Verona and Melody Bozzay in a research review for Policy Insights from the Behavioral and Brain Sciences.

Understanding those pathways is key to preventing violence, the researchers write. Verona and Bozzay’s review is part of a growing movement to analyze the brain circuitry involved in maladaptive behavior. After researchers began to see some of the same neurological pathways implicated in separate disorders, the National Institute of Mental Health developed a rubric for thinking about such common pathways, and Verona and Bozzay make use of it to identify two inter-related pathways that seem to be common to aggression across mental health diagnoses. The first is focused on affect, or emotion, and the second is focused on cognition, or thinking patterns.

When a person faces an acute threat of danger, brain circuits associated with emotions are activated, and this can lead to a defensive or angry emotional response and sometimes aggression. But many frightening situations don’t end in violence, of course, in large part because people usually have well-developed systems of cognitive control. The ability to think rationally, inhibit dangerous impulses, and exert self-control is associated with a part of the brain called the prefrontal cortex that is separate from the area involved in emotion. The pathways in that part of the brain are also affected by threat – their response is dampened at the same time the emotional ones are elevated – but the effect is usually temporary. However, when a person has intense or prolonged exposure to acute threat, that pathway may get triggered more often and lead to “sustained threat response” which involves hyper-reactivity, hyper-vigilance, and less cognitive control. This can lead to more impulsive reactions, and in some cases, the person may end up resorting to less sophisticated, or what the researchers call “evolutionarily older”, coping mechanisms, including aggression. And when imminent danger is very frequent, even the slightest suggestion of threat can set off this chain reaction.

One of the main ways to break this chain is to improve the environments in which we live. For example, the authors point to a study that showed people felt safer and less threatened when they were randomly assigned to live near vegetation as opposed to a more barren landscape. But many threats to personal safety are far too pernicious to be fixed by planting trees, and improving these environments is not necessarily a simple or quick process. Additional intervention targets are needed, especially ones that can be deployed immediately. Verona and Bozzay turn to research on the factors that can exacerbate or alleviate the neurological processes described above, focusing on one that may seem surprisingly simple: sleep.

Sleep loss dampens the functioning of the prefrontal cortex and therefore a person’s cognitive control. When people are sleep-deprived, studies show, they have heightened emotional responses, especially negative ones, and are more sensitive to stressors, even minor ones. As a result, insufficient sleep can initiate or exacerbate the cycle of hyper-awareness to threat that sometimes ends in aggression. And the relationship between aggression and sleep goes both ways, because acting aggressively often causes a person to feel depressed or anxious and have trouble sleeping. In other words, poor sleep and destructive behavior can become a vicious cycle. Insufficient sleep is a real public health problem, the authors point out, affecting about 30% of American adults, and it is “particularly common among the age groups involved in most acts of aggression and violence (aged 14 to 25 years old).”

Further study of the brain circuitry underlying aggression may help to identify additional targets for intervention. Research-driven strategies to prevent violence could benefit all of us, and the sooner the better.